The present invention relates to the fabrication of internal combustion engine parts from carbon-carbon composites. Carbon-carbon composites are made of carbon fiber reinforced in carbon matrix.
In the past, carbon-carbon parts have been used primarily in aeronautical and space applications because of their light-weight and high temperature properties. However, these characteristics are also extremely beneficial in industrial and automotive engines as evidenced by the fact that aluminum pistons(density 2.7 g/cm.sup.3), which are also lighter than steel(8.0 g/cm.sup.3), but heavier than carbon-carbon (1.7 g/cm.sup.3), have achieved significant commercial success in these markets. However, aluminum has a number of disadvantages. The relative difference in thermal strength and coefficient of thermal expansion of the aluminum pistons with other mating engine components, require large clearances between the piston and the adjacent walls to eliminate interference and galling between the piston and the cylinder wall and the wrist pin. To improve engine efficiency, piston rings are used in these aluminum pistons to seal the gap between the piston and the cylinder wall. In fact, multiple rings with staggering gaps are required to prevent high pressure leakage and possible piston erosion from local high flow rates at the rings, piston and cylinder wall inner face. Because of the poor, high temperature strength of aluminum, it has been found necessary to lower the piston rings from the crown to prevent the rings from sticking in the ring grooves, and this has resulted in unburned hydrocarbon build-up in the space around the piston above the ring yielding reduced engine efficiency, noting that aluminum melts at 660 degrees C. (and the maximum application temperature is 300 degrees C.), which is well below the typical combustion engine temperature. Also, large amounts of lubricant are required to reduce the piston and cylinder wall temperature and wear rates in aluminum piston assemblies.
The carbon-carbon components are desirable in this environment because of their resistance to high temperature and thermal shocks, coupled with high temperature strength. In some cases, the carbon-carbon piston can eliminate the necessity of piston rings because of the negligible coefficient of thermal expansion of carbon-carbon(1-2 ppm), which is far less than aluminum(18-20 ppm). Even at high temperatures, the carbon-carbon parts uniquely maintain strength, allowing the piston to operate at both higher temperature and higher pressure than metal pistons. Thermal efficiency of the engine is also improved because of the high emittance and low thermal efficiency of carbon-carbon, resulting in less heat loss into the piston and the cooling system.
The carbon fibers in the carbon-carbon composite are known as precursors, and there are three different types; namely, rayon, polyacrylonitrile, and pitch. Rayon has been largely abandoned in recent years because of the resulting poor quality fibers so that today fibers are predominantly made from P.A.N.(polyacrylonitrile) or pitch. P.A.N. is preferred for high strength, whereas pitch derivatives are desirable for high modulus and high thermal conductivity.
In reality, however, the use of carbon-carbon composites in engine components in the industrial and automotive market has not been extensive primarily for two reasons. The first is cost. In the early 1990's, carbon fiber used to cost about $40/lb., and now costs $8-$9/lb., and the near term projections are for under $5/lb. This cost reduction and an increased demand for fibers, which is projected, should drive the fiber cost down further making the carbon-carbon composites a very strong engineering material to replace steel and aluminum in many applications.
The second reason why carbon-carbon composites have not achieved great commercial success is the inability of fabricators to optimize and reduce the cost of the fabrication process. This is due in part to the difficulties in processing techniques to convert the binder to complete carbon which can hold the fibers, so the fibers therein reinforce the binder in such a way to have suitable engineering properties. Traditional processing consists of mixing the fiber with resin and preform into the desired shape. These preforms are kept in a high temperature furnace and heat treated for several hours ranging from 800 to 2000 degrees C. After firing, the composites are placed in a CVD furnace and densified. CVD refers to chemical vapor deposition. Due to the nature of CVD, it is extremely difficult to fabricate thick specimens with uniform density. As such, even for thin samples the CVD process takes from a few days to several weeks to finish. The CVD is sometimes replaced by chemical vapor infiltration (CVI), which causes carbon to close on the outside walls of the preform and inhibit penetration to the inside walls. Thus, in addition to time costs, the resulting crusting problem and its removal made these processes highly labor intensive and not conducive to high volume production.
In CVD, hydrocarbon gas is sent through the preform to crack it with high heat. This breaks the carbon down from hydrogen.
Another deficiency in the prior art of carbon-carbon high temperature components is the failure to optimize the performance of the component by controlling and varying the performance characteristics at specific locations on each part. For example, a rotating shaft under load will run hotter in the area of the bearings than at a point midway between the bearings. Prior art methodology for constructing such a shaft would result in homogenous physical properties throughout the shaft, and it is this approach that has in the past contributed to the high cost and less than optimum performance for carbon-carbon composites.
The Taylor, U.S. Pat. No. 4,683,809, assigned to NASA, shows a light-weight carbon-carbon piston with no piston rings. The piston is constructed in one piece and the fibers are laid up randomly throughout the piston. The methodology of fiber lay-up tends to disburse the fibers randomly resulting in internal cracks, unreliability, and low strength. The resulting piston component is heavy with poor fracture toughness. Taylor also suggests in this patent a carbon-carbon cylinder wall 60, but is silent as to how the cylinder wall or sleeve is formed or how its performance optimized.
Another Taylor patent, also assigned to NASA, is U.S. Pat. No. 4,736,676, which discloses a composite piston structure including a carbon-carbon or ceramic piston cap 11 with a metallic piston body 13. This piston is quite complicated and too difficult to manufacture in commercial production.
A later Taylor, et al., U.S. Pat. No. 4,909,133, also assigned to NASA, discloses a carbon-carbon piston that has a tubular closed ended knitted preformed sock of carbon fibers 11 imbedded within the matrix of the piston structure on the piston crown side wall in the inside surface.
Finally, the Fluga, U.S. Pat. No. 5,154,109, discloses a method of manufacturing a piston and piston rod in which a layer of carbon fibers is triaxially braided on a mandrel with a cylindrical body. A second layer of carbon fibers is triaxially braided over the first layer. The fiber layers are spaced from one another and impregnated with a thermo set resin. The preform is unidirectional in the sense that it does not have a uniform axial diameter. These are extremely difficult to manufacture and difficult to densify using CVD. Furthermore, the design is not flexible because the whole structure is made of one type of material, and thermal expansion is difficult to predict, and in some cases, may expand obliquely.
It is the primary object of the present invention to ameliorate the problems noted above and provide an improved method and structure for fabricating carbon-carbon light-weight components intended for high temperature environments.